This invention relates to infrared photoconductor arrays and more particularly to a method of assembly-line quality selection of thermally optimized photoconductive arrays.
It is well recognized that, during miliary use of electro-optical systems, the electro-optical elements may be irradiated by high-power laser beams. Thus, it is desirable to develop systems which will function in such an environment. In developing such systems, it is necessary that electro-optic arrays be checked to determine whether or not their operation is within required ranges.
It has been determined that a primary limitation of photoconductor array performance under high thermal loading is the inefficient thermal coupling between the heat-absorbing detector elements and the heat sink. In present state-of-the-art IR photoconductor arrays, the individual detectors in the array are bonded to a substrate by use of epoxy and the substrate is usually mounted on a cyrogenic heat sink by means of an adhesive layer such as a low-temperature varnish. The two bonding layers are thermally resistive and act as bottlenecks limiting the flow of heat from the detector elements to the heat sink.
In the fabrication of such IR detector arrays, wide variations in the thermal conductance between the detector element and the heat sink are observed due to imperfections in the bonding layers. It is necessary therefore to provide a technique for quality control and selection whereby the thermal conductances of the bonding layers may be measured in situ for each detector of an array as well as for each of the arrays of the assembly line.
Present techniques for measuring the thermal characteristics are restricted to DC bias-current effects. The main limitation to this approach is that it provides the thermal conductance of each detector in the array, but not of the individual layers comprising each detector of the array. Such a test provides information on steady-state power dissipation for a given detector but not on the thermal response of the detector to pulsed radiation.
Heretofore a method has been set forth in U.S. Pat. No. 4,012,691 for independently determining the thermal conductances of the two bonding layers of single-element detectors. In that technique the detector signal is measured during and after a laser pulse and is found to fall abruptly upon removal of the incident laser radiation; however, the signal does not completely return to its dark bias level until the resistance of the heated detector returns to its initial value. The recovery times are characteristic of heat transfer through the various layers of material. The magnitude of the thermally induced signal, the relative importance of the recovery processes and the exact shape of the thermal curve vary greatly with power density and irradiation time. Temperature profiles calculated for a laser-irradiated detector show that the bonding layers used to mount the IR detector on the substrate and the substrate on the heat sink are the two major factors limiting the performance of infrared photodetectors under high thermal loading.
Comparison of experimental thermal recovery curves with theoretical curves calculated by using a one-demensional thermal model allows one to determine the thermal properties of these two bonding layers. The above technique is successful for single IR detectors; however, the technique is not applicable for detector array assemblies. In single IR detectors, heat flow is essentially one-demensional, that is, heat flows in one direction from the IR detector material, through the epoxy bonding layer to the substrate and through the varnish layer to the heat sink. For detector arrays, heat flows not only in the direction from the detector material to the heat sink, but also perpendicular to that direction within the substrate. Because of this perpendicular component of heat flow, the technique set forth in U.S. Pat. No. 4,012,691 is not applicable to detector arrays and a more sophisticated approach to this thermal problem is required. Differences in the two systems have been set forth in a publication, "Thermal Conductance of Bonding Layers in Hgcdte (PC) Detector Arrays," by F. Bartoli et al., Applied Optics Vol. 15, No. 9 pp. 2016-2017, September 1976, which is incorporated herein by reference.